I would not soak a branch in bleach. you might kill bacteria, but you would be very unlikely to get rid of all the bleach, which will kill your gecko faster than most bacteria. it is very, very difficult to remove all the bleach from something so porous.
also, 300F for 10 min won't kill much of anything
I've been using branches in my vivs for about 5 years, for Cresties, Mad-Mads, and L williamsi (+ a few python species and an occasional frog).
first, what species of branch? if it's from anything pine (especially cedar) it's toxic - toss it out.
I don't have a comprehensive list of toxic trees, although there are some great lists out there of plants that are / are not safe. these are the ones I have personally used and have found to be fine, at least for several years. I can't vouch for longterm liver effects yet.
-maple
-manzanita
-madrona
-apple
first off, I rinse branches very, very thoroughly. I leave the bark on. I then let them dry for a long time. outside is good if it's sunny where you live. I used to put the branches on my front porch, up off the ground (fewer bugs and stuff) and in sunlight much of the day.
watch for bugs and stuff to crawl out of the wood. maybe peel up a little bark at one end and inspect for bugs underneath.
then I repeat the thorough rinsing, this time lightly going over the bark with a fingernail brush.
then I stick them in the cage. with about 10 vivariums or so, I've only had an issue once - I had slug larvae hatch in one of my cages. they probably came in on a branch, although they also may have come from the rocks I used.
from a microbiological standpoint, you can't really get a branch hot enough in your oven to kill stuff that's most likely to be harmful - spore-forming bacteria and fungi -without damaging your branch. also, these guys live with bacteria in the wild, and bacteria are important to form a healthy ecosystem in the tank.
however - where I live, there are very few native herps, and I've never seen any on or near my property. I'd be more concerned about transmittable disease if I lived somewhere with a big wild herp population. also, no pesticides or scary chemicals were used in my yard. you don't want that stuff in your vivs, either.
if you're really concerned about bacteria, I'd just get a couple cans of sprayable water-based polyurethane, spray the hell out of the branch, cure/off-gas thoroughly for several days, then use it like that. anything icky will be sealed inside, and you'll retain most of the texture of the bark.